As a fast long distance option, or in a map with a really really difficult topography. But you are mostly right, distances aren't that large, so it is a little bit like a luxury item.

Cargo airports are cheap and can be used as a more fuel-efficient way to access the western markets when you didn't start near a westerns customs house or with water access to them.

In the late game they are good for minimizing the buildup of expensive goods in your transport networks, mainly due to the speed of airplanes. They are also good for quickly getting materials or goods to an area that lacks them. So if a town is about to run out of food, you can quickly fly in some food to prevent starvation.

If you know what you are doing, you can also use them to somewhat quickly and cheaply expand into areas that are far away (8+ km) that lack access to water or nearby customs houses. You just need like 6 cargo helicopters and some skill to build a cargo airport about ~10 km away in less than a year. Gravel and then asphalt & concrete can be made near the airport, while AN-12's haul in almost everything else. Coal and fuel will still need to be manually brought in by helicopters though.